integrated circuits

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MODULE 4.1.3. INTEGRATED uk engineering JAR 66 CATEGORY B1 CONVERSION COURSE MODULE 4 ELECTRONIC 1 INTEGRATED CIRCUITS 1.1 GENERAL Integrated circuits, or IC’s, have changed the entire electronics industry. Before IC’s were developed, all electronic circuits consisted of individual (discrete), components that were wired together, often requiring a large amount of physical space. Printed circuit Board (PCB) technology made it possible to reduce the amount of space required. Electronic circuits can be quite complex, requiring a large number of components, since discrete components have a fixed size, there is a practical limitation on the amount of size reduction that can be achieved. The development of integrated circuit technology has made it possible to fabricate large numbers of electronic components onto a single silicon chip. As a result, the physical size of a circuit can be significantly reduced, making it possible to design circuits and devices that would otherwise be impractical. IC’s are complete circuits containing many transistors, diodes, resistors and capacitors as may be necessary for the circuit operation. They are encapsulated in packages that are often no larger than a single discrete transistor. The technology and materials used in the manufacture of IC’s are basically the same as those used in the manufacture of transistors and other solid-state devices. In addition, IC’s are manufactured for a wide variety of applications and, as a result, are used throughout the electronics industry. 1.1.1 ADVANTAGES The small size of the IC is its most apparent advantage. A typical IC can be constructed on a piece of semiconductor material that is less than 4mm 2 . Even when the IC is suitably packaged, it still occupies

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Integrated Circuits

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Page 1: Integrated Circuits

MODULE 4.1.3.

INTEGRATED ukengineering

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1 INTEGRATED CIRCUITS

1.1 GENERAL

Integrated circuits, or IC’s, have changed the entire electronics industry. Before IC’s were developed, all electronic circuits consisted of individual (discrete), components that were wired together, often requiring a large amount of physical space. Printed circuit Board (PCB) technology made it possible to reduce the amount of space required. Electronic circuits can be quite complex, requiring a large number of components, since discrete components have a fixed size, there is a practical limitation on the amount of size reduction that can be achieved.

The development of integrated circuit technology has made it possible to fabricate large numbers of electronic components onto a single silicon chip. As a result, the physical size of a circuit can be significantly reduced, making it possible to design circuits and devices that would otherwise be impractical.

IC’s are complete circuits containing many transistors, diodes, resistors and capacitors as may be necessary for the circuit operation. They are encapsulated in packages that are often no larger than a single discrete transistor. The technology and materials used in the manufacture of IC’s are basically the same as those used in the manufacture of transistors and other solid-state devices. In addition, IC’s are manufactured for a wide variety of applications and, as a result, are used throughout the electronics industry.

1.1.1 ADVANTAGES

The small size of the IC is its most apparent advantage. A typical IC can be constructed on a piece of semiconductor material that is less than 4mm2. Even when the IC is suitably packaged, it still occupies only a small amount of space. The small size of the IC also produces other benefits such as they consume less power than the equivalent conventional circuit. They generate less heat and therefore generally do not require elaborate cooling or ventilation systems.

IC’s are also more reliable than conventional circuits. This greater reliability result because every component within the IC is a solid-state device and is permanently connected together with a thin layer of metal. They are not soldered together like the components in a conventional circuit and a circuit failure due to faulty connections is less likely to occur.

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1.1.2 DISADVANTAGES

It might appear that the IC has only advantages to offer and no real disadvantages. Unfortunately, this is not the case, since IC’s are an extremely small device it cannot handle large currents or voltages. High currents generate heat within the device and small components can be easily damaged if the heat becomes excessive.

High voltages can break down the insulation between the components in the IC because the components are very close together. This can result in shorts between the adjacent components, which would make the IC completely useless. Therefore, most IC’s are low power devices, which have a low operating current (milliamps) and low voltages (5 – 20V). Also, most IC’s have a power dissipation range of less than 1 watt.

At the present only four types of component are commonly constructed within an IC. This makes only a narrow selection of components available, these are:

1. Diode.

2. Transistor.

3. Resistor.

4. Capacitor.

Diodes and transistors are the easiest components to construct and are used extensively to perform as many functions as possible within each IC. Resistors and capacitors may also be formed, but it is much more difficult and expensive to construct these components. The amount of space occupied by a resistor increases with its value and in order to conserve space, it is necessary to use resistors with values as low as possible.

Capacitors occupy even more space than resistors and the amount of space required increases with the value of the capacitor.

Ic’s cannot be repaired because their internal components cannot be seperated. When one internal component becomes defective, the whole IC becomes defective and musty be replaced. This means that good components are often thrown away with the defective ones. This disadvantage is not as bad as it sounds, as the task of fault finding is simplified because it is only necessary to trace the problem to a specific circuit instead of an individual component. This greatly simplifies the task of maintaining highly complex systems and reduces the demands on maintenance personnel.

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1.2 IC CONSTRUCTION

There are basically four methods of construction used for IC’s. These are:

1. Monolithic.

2. Thin-Film.

3. Thick Film.

4. Hybrid.

1.2.1 MONOLITHIC IC’S

The monolithic IC is constructed in basically the same manner as a “Bipolar Transistor”, although the overall process requires a few additional steps because of the greater complexity of the IC. Its fabrication begins with a circular semiconductor wafer (usually silicon). This wafer is usually very thin (0.015mm – 0.3mm) and either 2.5cm or 5cm in diameter. The semiconductor serves as a base on which the tiny integrated circuits are formed and is commonly referred to as a “Substrate”. Figure 1 shows the IC construction.

IC Construction

Figure 1

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When all of the IC’s have been simultaneously formed, the wafer is sliced into many sections, which are commonly referred to as “Chips” or “Dice”. Each chip represents one complete integrated circuit and contains all the components and wiring associated with that circuit. Once the IC’s have been separated into individual chips, each IC must be mounted in a suitable package and tested.

1.2.2 BIPOLAR IC CONSTRUCTION

As mentioned earlier, the components that are commonly used in IC’s are diodes, Transistors, resistors and capacitors. Diffusing impurities into selected regions of a semiconductor wafer (substrate) can form these components. This process produces PN junctions at specific locations and the basic manner in which these four components are formed and the manner in which they are interconnected are shown at Figure 2.

Basic Construction of Bipolar ICFigure 2

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The circuit shown in figure 2 is a simple circuit consisting of a capacitor, a PN junction diode, an NPN transistor and a resistor. Operating voltages and currents can be applied to the circuit through terminals 1,2 and 3 as shown. This circuit could be easily constructed using four discrete components, however, it can also be produced as a monolithic IC.

1.2.3 MOS IC’S

Not all IC’s are constructed using bipolar components, IC’s are often designed to utilize either bipolar transistors or “Field-Effect transistors” (FETS). The Field effect transistor is one in which the emitter-collector current is controlled by voltage rather than by a current. Figure 3 shows the construction and operation of a MOSFET.

MOSFETFigure 3

The FET may be constructed of a channel of either N-type or P-type silicon with a controlling gate sitting on top. One end of the channel is called the source, and the other end is called the drain. An N-channel FET has a P-type gate, so that when a positive voltage ios applied to the gate, the FET is forward biased. There will be current flow between the source and the drain. When a negative voltage is applied to the gate, the FET will be reversed biased, and the flow between the source and the drain will be pinched off.

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The source and drain regions are diffused into the substrate. A thin layer of silicon oxide is formed over the substarte and the appropriate windows are cut into it so that metal electrodes ) terminals) can be formed at the proper locations. Note that the gate terminal is separated from the substrate by an extremely thin oxide layer, which is only 1 X 10-10 metres thick, but it completely isolates the gate from the substrate.

1.2.4 THIN-FILM IC

Unlike the monolithic IC’s, which are formed within a semiconductor material (substrate), the thin-film circuit is formed on the surface of an insulating substrate. In the thin-film circuit, components such as resistors and capacitors are formed from extremely thin layers of metals and oxides, which are deposited onto a glass or ceramic substrate. Interconnecting wires are also deposited on the substrate as thin strips of metal. Components such as diodes and transistors are formed as separate semiconductor devices and then permanently attached to the substrate at the appropriate locations.

The substrate on which the thin-film circuit is formed is usually less than 2.5cm2. Depositing tantalum or nichrome as thin films or strips on the surface of the substrate forms the resistors. These films are usually less than 0.00254cm thick. The thickness, length and width of each strip that is formed on the substrate determine the value of each resistor. The interconnecting conductors are extremely thin metal strips, which have been deposited on the substrate. Low resistance metals, such as gold. platinum, or aluminium, are generally used as conductors. The substrate is made from an insulating material that will provide a rigid support for the components. Glass or ceramic materials are often used as substrates. Figure 4 shows a portion of a thin-film circuit.

Thin-Film ICFigure 4

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1.2.5 THICK-FILM IC’S

Thick-film IC’s components are formed on an insulating substrate by using a “Silk-screen” process. In this process, a very fine wire screen is placed over the substrate and a metalized-ink is forced through the screen using a squeegee. Only certain portions of the wire screen are open (the remaining portions are filled with a special emulsion), thus allowing the ink to penetrate and coat the specific portions of the substrate. A pattern of interconnecting conductors is formed on the substrate, which is then heated to over 6000C to harden the painted surface and become low resistance conductors.

Resistors and capacitors are also silk-screened on top of the substrate by forcing the appropriate materials (in paste form) through the appropriate screen and then heating the substrate to a high temperature. This process is repeated using various pastes until the circuit is formed. Components such as diodes and transistors are formed as separate semiconductor devices and then permanently attached to the substrate at the appropriate locations.

1.2.6 HYBRID IC’S

Hybrid IC’s are formed by utilizing various combinations of monolithic, thin-film and thick film techniques and may in certain circumstances contain discrete semiconductor components in chip form. Therefore many types of hybrid circuit arrangements can be produced. A typical hybrid circuit might consist of a thin-film circuit on which various monolithic IC’s have been attached or it could utilize monolithic IC’s thick-film components and discrete diodes and transistors that are all mounted on a single insulating substrate.

A portion of a hybrid IC is shown at figure 5. An insulated substrate is used to support the circuit components as shown. The monolithic IC is mounted on the substrate along with thich-film resistors and a small discrete capacitor. All the components are interconnected with conductors that are formed on the substrate using film techniques. The monolithic IC is connected to the conductors with fine wires that are bonded in place. Thick-film resistors will usually have notches cut into them to trim their values. The capacitor used in these circuits can be formed either by using film techniques or miniature devices can be installed between conductors as shown.

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Hybrid IC ConstructionFigure 5

1.2.7 IC PACKAGES

Like transistors and other types of solid state components, IC’s are mounted in packages, which protect them from moisture, dust and other types of contaminations. Many different types of IC packages are available and each type has its own advantges and disadvantages. The most popular IC package is the “Dual In-Line (DIL) package. The packages also make it easier to install the IC’s in various types of equipment, since each package contains leads which can be either plugged into matching sockets or plugged into DIL mounting frames.

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Figure 6 shows typical DIL packages.

DIL PackagesFigure 6

The IC package shown in figure 6 contains three monolithic IC’s, also a network of conductors have been formed on the same base that supports the chip. Various conductor pads on the chips are connected to these conductors with fine gold wires that have been bonded in place. The conductors in turn are connected to two rows of connecting pins along the edge of the package. A lid or cover (not shown) is placed over the opening in the package and soldered into place to provide an air tight (hermetically sealed) unit.

Integrated circuits may also be mounted in “Metal cans” that are similar to the types used to house transistors. The metal can have 8 or more connecting leads and can used to house either monolithic or hybrid type IC’s. The advantage of these packages is that they may be installed in a variety of ways. Metal cans can be used over a wide temperature range (-55 - +125C) and are therefore suitable for military and space applications. Figure 7 shows the DIL and metal can type of packages.

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DIL and Metal Can PackagesFigure 7

1.3 TYPES OF INTEGRATED CIRCUIT

Integrated circuits are placed into two general groups, these are:

1. Digital IC’s.

2. Linear IC’s.

1.4 DIGITAL IC’S

Digital circuits use discrete values (0 or 1) to perform 3 general functions. These are:

1. AND Function.

2. OR Function.

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3. NOT Function.

Thgese three function are performed by logic circuits that are called the AND, OR and NOT logic gates. These gates or circuit configurations can be combined to make decision based on digital input information. In a digital logic gate it is only possible to have an output of either a 0 or 1.

1.4.1 AND GATE

Figure 8 shows the AND gate truth table and logic circuit and a corresponding circuit to carry out this function.

AND GateFigure 8

The AND gate has an output of 1 only when all of its inputs are equal to 1. This is similar to a multiplier function since the only possibilities in a digital circuit are 0 X 1 = 0 and 1 X 1 = 1. The schematic circuit in figure 8 shows two switches connected in series. Unless both switches are closed, there is no current flow to the output.

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1.4.2 OR GATE

Figure 9 shows the OR gate truth table and logic circuit and a corresponding circuit to carry out this function.

OR GateFigure 9

1.4.3 NOT GATE

The NOT gate provides an output that is always the opposite the input. This is called inversion or 180 phase shift. Thus, the NOT gate is commonly referred to as an inverter. In the bipolar transistor, the common emitter amplifier configuration was the only one capable of inverting the input so is used to carry out the NOT function.

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Figure 10 shows the NOT gate truth table and logic circuit and a corresponding circuit to carry out this function.

NOT GateFigure 10

1.4.4 COMBINATION LOGIC CIRCUITS

The three basic logic circuits can be combined into a single decision making circuit with more than 1 distinct outputs. Consider a circuit that compares two inputs and calculates three outputs as shown below.

Output X1 Input A < Input B

Output X2 Input A > Input B

Output X3 Input A = Input B

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A combined logic circuit that would carry out the function is shown at Figure 11.

Combination Logic Circuit

Figure 11

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1.5 LINEAR (OR ANALOGUE) IC

Figure 12 shows the type of analogue signal handled by the Linear Integrated Circuit.

Analogue SignalFigure 1

1.6 THE OPERATIONAL AMPLIFIER (OP AMP)

The integrated circuit operational amplifier is one of the most useful and versatile electronic devices available today. The name ‘operational amplifier’ is not new; it refers to a type of amplifier originally used in analogue computing to perform mathematical operations – e.g. multiplication or division by a constant. The modern integrated circuit device can be adapted (by feedback) to perform most general-purpose amplifier duties, as well as its use in mathematical operations.

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The Op Amp can consist of many stages of amplification to ensure high gain, and will be arranged to have two input terminals, two power supply terminals and an output terminal. In addition it will normally have terminals for setting the output to zero when the input is zero.

The Op Amp consists of a transistor circuit of considerable complexity, which has been found so useful that the whole circuit is manufactured on a single piece of silicon, fitted with input and output leads, and covered in plastic. It is the first “Integrated Circuit”, and can be treated just as if it were a new component. Figure 2 shows a type 741 Op Amp and circuit.

Op Amp and CircuitFigure 2

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In the Op Amp, two pins are marked supply + and supply - and are connected to the amplifiers power supply. The device also has two inputs, the “Inverting input” (V) identified by a negative symbol. A “Non inverting input” (VN) identified by a positive sign and a single output (VO).

Note: The negative/Positive signs on the inputs does not mean that negative/positive signals are applied, but identify the inverting and non-inverting terminals.

The V, VN and VO are the values of the voltages applied to the inputs and obtained form the output. These voltages are joined by the equation:

VO = AO (VN – V)Here we have a slight problem. Voltages are measured between one point in a circuit and another. Usually one point is the negative or zero line. When calculating VN & V it does not matter were the reference is as long as it is the same for both voltages. When we obtain the output VO we need to know the reference point used by the Op Amp. This is not the zero line but a voltage halfway between the positive supply and the zero line.

The other unknown quantity in the equation is AO, the “Open Loop Gain”. This gain is constant for each particular Op Amp and is the ratio between two voltages. Open Loop gain in Op Amps is normally 105.

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The following example will make use of the equation. Figure 3 shows an Op Amp with an open loop voltage gain of 400, connected between a 12V supply.

Op AmpFigure 3

V = 5.88V VN = 5.87 AO = 400Using the equation:

VO = AO(VN - V)VO = 400(5.87 – 5.88)

= 400(-0.01)= -4V

The voltage is relative to a point halfway between +12v and zero, that is 6V. The output voltage is therefore 4V below 6V, i.e. 2V. What would the output be if the input values were reversed?

Ans:……………………….

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1.7 THE UNITY GAIN FOLLOWER

The somewhat lengthy term operational amplifier will now be dropped and it will be referred to, as most people do, simply as the op amp. The simplest op amp with feedback configuration is the unity gain follower, which has a direct connection from output to inverting terminal, and the input is applied to the non-inverting terminal as shown in Figure 4. For convenience, power supplies will not be shown in these circuits.

Unity Gain FollowerFigure 4

Consider the circuit in Figure 4 and assume that vin is a small positive dc voltage. With its enormous gain the output will probably saturate positive as soon as v in is applied. But the output is connected directly to the input inverting terminal, and being much greater than vin will drive the output to saturate negative, which is fed back to the input inverting terminal to drive the output to saturate positive again. It would appear that the output voltage is slamming from positive saturation to negative saturation and back again at a rapid rate. This of course is not the case at all, since in its travel from one extreme to the other, the output will arrive at the same value as the input, and when this is fed back to the inverting input terminal, the input difference voltage will be zero and there will be no signal to amplify. Thus vout holds a value equal to vin.

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What has happened here is very important, since it applies to all op amps with negative feedback circuits, and it should be noted that feedback from the output to the inverting input terminal causes the output to take on a value which reduces the input difference voltage to zero.

Since the output of the unity gain follower is the same as the input, it may, on first consideration, have no practical application. As previously stated, even if the feedback caused the output to equal the input, the input and output impedances remain vastly different. Herein lies its practical application, as an impedance matching device, and it serves as an excellent ‘buffer’ stage preventing interaction between a signal source and load.

1.8 THE FOLLOWER WITH GAIN

By reducing the amount of voltage fed back to the inverting terminal, the gain of the amplifier can be raised above unity, and effectively multiply the input by a factor determined by the amount of feedback. The reduction in feedback voltage may be achieved by a potential divider arranged as shown in Figure 5.

Follower with GainFigure 5

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1.9 THE INVERTER

The circuit incorporates a sign change between input and output. This is achieved by bringing the input into the inverting terminal along with the feedback loop, and by earthing the non-inverting terminal, as shown in Figure 6. The resistors R1 and R2 still maintain the multiplication (or division) factor.

The InverterFigure 5

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1.10 THE SUMMING AMPLIFIER

As the title implies, this is an adding device. The circuit can take any number of voltage inputs, and the output voltage is simply the sum of these inputs.

The circuit in Figure 7 has a feedback loop containing a resistor Rf and the facility to sum three input voltages.

Summing AmplifierFigure 7

It must again be emphasised that using negative feedback, the feedback loop forces the output to take on a value, which reduces the input difference voltage to zero.

The input voltages V1, V2 and V3 are supplied to the inverting (-) terminal via resistors R1, R2 and R3. Since the input difference voltage is zero, the potential at the inverting terminal and hence the junction of R1, R2 and R3 is the same as the non-inverting (+) terminal, which is earth. Also, since the inputs are applied to the inverting terminal, the output voltage will experience an inversion (sign change).

The output of the circuit, then, is the sum of the input voltages. Note again that inversion takes place due to the inputs being applied to the inverting terminal.

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1.11 THE DIFFERENCE AMPLIFIER

Figure 8 shows how two inputs can be subtracted. This circuit differs from previous types with negative feedback in that the non-inverting terminal is not at earth. As before, however, the negative feedback drives the output to a value, which reduces the input difference voltage to zero.

Difference AmplifierFigure 8

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1.12 THE INTEGRATOR

This circuit performs the calculus operation of integration. An operational amplifier with negative feedback applied via a capacitor (instead of a resistor) will perform the mathematical operation of integration (see Figure 8). Such circuits are widely used in analogue computing - e.g. if the input voltage uses the analogue of acceleration (m/s/s), the output voltage is the analogue of speed (m/s). Another use of this type of circuit is to produce a triangular (or ramp) waveform from a square wave (or step) input; this technique is used in digital voltmeters and some forms of analogue/digital converters. Figure 9 shows an Integrator.

IntegratorFigure 9

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1.13 THE DIFFERENTIATOR

With negative feedback applied through a resistor and the input signal through a capacitor, the circuit will perform the mathematical operation of differentiation. The output voltage then takes up a value proportional to the rate of change of the input voltage - e.g. if the input voltage is the analogue of distance travelled (miles), the output is the analogue of average speed (miles per hour). The circuit can also be used to produce ‘pips’ (e.g. calibration markers) when the input signal is a square wave. (To prevent high frequency instability, it is usual to connect a small capacitor (e.g. 10 F) across the feedback resistor to reduce gain at frequencies above the required operating frequency). Figure 10 shows a differentiating circuit.

Differentiating CircuitFigure 10

As in all previous circuits with negative feedback loops, the output voltage takes on a value, which reduces the input difference voltage to zero, and a virtual earth exists on the inverting terminal. Vin is across C and -vout across R.

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1.14 THE COMPARATOR

A comparator is a device, which compares two input voltages and indicates at the output, which of the two is the larger. Basically, the circuits use a differential input operational amplifier without feedback: so the output takes up either the positive or negative supply level depending on which of the inputs is higher. See Figure 11.

The ComparatorFigure 11

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1.15 THE IDEAL OPERATIONAL AMPLIFIER

Although the characteristics of an ideal operational amplifier are unattainable, modern integrated circuit types can provide an extremely close approximation.

The ideal characteristics are:

* A very large open loop gain, near infinite,

* Output unaffected by signal frequency, no signal phase shift with change in frequency,

* A very large (infinite) input impedance so that the amplifier takes negligible current,

* A very small output impedance so that the output of the amplifier is unaffected by loading,

* Output voltage is zero for zero input voltage (offset zero applied).

Naturally, no practical operational amplifier will be this perfect, which means of course that there will be small operational errors with such devices. Therefore, the closer to the ideal properties the amplifier is made, the smaller will be these errors.

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